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Polaris

The Pole Star, The North Star, Cynosura,Alpha Ursae Minoris, 1 Ursae Minoris

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 Proper Names Cynosura, North Star, Polaris, Pole Star Bayer Designation Alpha Ursae Minoris Flamsteed Number 1 Ursae Minoris BSC 424 HD 8890 Constellation Ursa Minor Right Ascension 2h 35m 54s Declination +89° 16' 49" Distance 431 light years132 parsecs Magnitude Apparent: +2.1Absolute: -3.5 Spectral Class F7 Bright Yellow Supergiant Optimum Visibility Always visible from northern latitudes

Polaris, the famous Pole Star, lies almost exactly at the Northern Celestial Pole.

A view of the pulsating supergiant Polaris. In the foreground is a much less massive dwarf star, one of several companions of the immense yellow Pole Star.

The Galactic position and direction of Polaris relative to Earth's Sun. Note that, at this extreme scale, the two stars are effectively in the same place.

The famous Pole Star lies less than one degree from the Northern Celestial Pole, and so always lies north from an Earth-bound observer's point of view. Its name comes from Latin, Stella Polaris, meaning simply 'Pole Star'. Its alternative and much rarer name, Cynosura, comes from the Greek for 'tail of the dog'.

As the Earth turns on its axis, the stars in the sky seem to turn around the Northern Celestial Pole. This image shows the paths traced by the stars as the swarm around the Pole over a period of eight hours. As the nearest bright star to the Pole, Polaris appears as the small bright crescent in the centre of the image.

Physically, Polaris is very massive star of the 'F' (bright yellow) classification. It is particularly notable as being one of the nearest Cepheid variables to Earth; it is less then half the distance of Delta Cephei itself. This is important because Polaris' four-day cycle of swelling and contraction is directly related to its mass, which is in turn related to its luminosity. This means that we can calculate the star's absolute magnitude with some certainty, and comparing this with its observed apparent magnitude, we can compute its distance. Polaris' mean absolute magnitude is -3.50, and its mean apparent magnitude is +2.11, which gives us a distance of just over 431 light years.